I have to admit my knowledge of the secret NPCs in Undertale isn’t exactly up to snuff, but is it totally out of the question that the Suzy mentioned by the NPC in Waterfall has some connection to Deltarune’s Susie? I found it really strange that during the chapter 2 epilogue segment you can read all of the gravestones except the grave for the Shyren-related monster (maybe the amalgamate Lemon Bread?), which prompts Susie to try to hurry you out of the cemetery, preventing you from reading it at all… implying she has some kind of Waterfall family history?